For example, Japanese Laid-open Patent Publication No. 6-224652 discusses a light reception circuit in which bias elements are coupled to both ends of a photodiode, an anode and a cathode of the photodiode are coupled to input terminals of a differential amplification unit via capacitors, and resistors are coupled between a positive input terminal and an inverting output terminal of a differential amplifier and between a negative input terminal and a positive output terminal of the differential amplifier.
For example, Japanese Laid-open Patent Publication No. 2008-180522 discusses a photoelectric encoder in which a direct current (DC) offset of a light reception signal is reduced based on the fluctuating light reception signal itself by negatively feeding back a DC output of a differential amplifier to an input side of diodes making up a logarithmic compression unit.
For example, Japanese Laid-open Patent Publication No. 2002-232271 discusses a DC offset cancel circuit that cancels a DC offset voltage caused in a differential output signal of a differential amplification circuit by negatively feeding back the differential output signal of the differential amplification circuit to an input side of the differential amplification circuit via a low-pass filter.
For example, Japanese Laid-open Patent Publication No. 2001-168374 discusses a photoelectric conversion circuit in which resistors are coupled to a cathode and an anode of a photodiode respectively, another resistor is coupled in parallel to the photodiode, the cathode and the anode of the photodiode are coupled to two input terminals of a level shift circuit respectively, and output signals are obtained from two output terminals of the level shift circuit. In the photoelectric conversion circuit, a source follower circuit is used as the level shift circuit.
In order to achieve high speed processing in an arrangement in which an anode and a cathode of a photodiode and each input terminal of a differential amplifier are coupled by capacitive coupling via a capacitor, an input impedance of the differential amplifier is desirably lowered. However, when the input impedance of the differential amplifier is lowered, low-frequency cutoff may be caused by the capacitor and fluctuations may occur in a reception waveform of a signal formed of “0” and “1”, which are continuous values, and as a result, the waveform may deteriorate.